Smooth muscle contraction is essential to the normal function of many organ systems within the body. Therefore, an understanding of its normal contractile process is required before studying disease states such as hypertension in which smooth muscle function may be abnormal. The ability of smooth muscle to sustain prolonged isometric contractions with very little energy consumption (i.e. ATP) may relate to the most basic contractile unit, the myosin crossbridge, and its cyclic interaction with actin. Although the smooth muscle crossbridge cycle may be qualitatively similar to that in skeletal muscle, its mode of regulation is quite different. Specifically, phosphorylation of smooth muscle's 2OkD myosin light chain initiates contraction and crossbridge cycling. However, the degree of light chain phosphorylation may also modulate the crossbridge cycling rate. To characterize the effect of light chain phosphorylation on the crossbridge cycling rate, a motility assay has been developed in which the motion of single fluorescently labelled actin filaments will be measured as these filaments are propelled by synthetic smooth muscle myosin filaments containing known proportions of dephosphorylated and phosphorylated crossbridges. In addition, techniques will be developed for recording force sustained by a single actin filament as it interacts with a myosin coated glass coverslip. The motility assay provides a unique opportunity to probe the molecular regulation of smooth muscle contractile proteins.